Host cell protein synthesis

Many Viruses Co-opt the Host Cell Protein Synthesis Machinery... 

One subunit (Mr 65,000) is the product of the replicase gene encoded by the viral RNA and has the active site for replication. The other three subunits are host proteins normally involved in host-cell protein synthesis the E. coli elongation factors Tu (Mt 30,000) and Ts (Mr 45,000)... 

Achneider, R. J., and T. Shenk, Impact of virus infection on host cell protein synthesis. Ann. Rev. Biochem. 56 317— 332, 1987. 

Ehrenfeld, E. (1982). Poliovirus-induced inhibition of host-cell protein synthesis. Cell 28, 435-436. 

Two other viral vectors are extensively used. Vaccinia virus, a large DNA-containing virus, replicates in the cytoplasm of mammalian cells, where it shuts dovm host-cell protein synthesis. Baculovirus infects insect cells, which can be conveniently cultured. Insect larvae infected with this virus can serve as efficient protein factories. Vectors based on these large-genome viruses have been engineered to express DNA inserts efficiently. 

The fate of HSV-1 infection can be either lytic or latent. During the lytic replication cycle, which occurs in mucosal or epithelial cells, the host cell protein synthesis is shut off by the tegument vhs protein immediately after infection. Subsequently, the viral capsid releases the viral DNA into the nucleus where it will circularize. 

Poliovirus is an RNA-RNA virus that redirects the host-cell protein synthesis machinery toward translation of its own RNA genome. The poliovirus polypeptide is cleaved into active subunits by the proteolytic action of virally encoded proteases. Poliovirus RNA replication utilizes a novel protein-uridine primer to initiate RNA synthesis using its own RNA-dependent RNA polymerase. 

Apparently related to the problem of host cell protein synthesis inhibition by viruses is the phenomenon of viral interference. In some cases, superinfection of a cell already infected with one virus does not affect the yield of the original virus. In other cases, however, the superinfecting virus completely inhibits translation of the original viral mRRA. Such is the case for superinfection of vesicular stomatitis virus (VSV)-infected cells by poliovirus (32). The kinetics and general properties of the shut-off of VSY protein synthesis appear to be the same as the shut-off of cellular protein synthesis after poliovirus infection... 

WILLEMS, M. and PENMAN, S. The mechanism of host cell protein synthesis inhibition by poliovirus. Virology, (I966), 30<... 

CARRASCO, L. and SMITH, A.E. Sodium ions and the shut-off of host cell protein synthesis by picornaviruses. Nature (London), (1976), 2, 8O7-8O9. 

Extracts prepared from infected cells may be a useful tool to investigate the mechanism of the shut-off of the host-cell protein synthesis. In the case of picornaviirus-infected cells, however, the cell extracts are relatively inactive in protein synthesis unless supplemented with initiation factors. This inhibition of protein synthesis in cell extracts is probably caused by dsENA of viral origin. 

Ribosomes phosphorylated vitro show no alteration in their translational activity (5, ) Trivial explanations such as the possibility that the ribosomes are dephosphorylated in the translation assay seem to have been eliminated. It is, of course, possible that current assay methods are inadequate to detect subtle changes caused by phosphorylation, but it is equally valid to draw the strai tforward conclusion that phosphorylation does not affect the activity of ribosomes. This is in accord with the fact that changes in the phosphorylation state of ribosomes vivo do not appear to correlate with alterations in the efficiency of the ribosomes in protein biosynthesis. The one possible exception is the phosphorylation of one of the small ribosomal subunit proteins (S2) that occurs after infection by vaccinia virus (8). The timing of the phosphorylation of S2 seems however to be related neither to the shut-off of host cell protein synthesis nor to the switch from early viral protein synthesis to late gene expression, and it remains to be proven whether the phosphorylation has any material effect on ribosome activity or specificity. 

The other view, which is not necessarily in contradiction,stems from the observation that in a competitive situation the translation of EMC RNA outcompetes the translation of cellular mRNA in vitro under virtually all experimental conditions (56). The only exception is that the addition of extra eIF-4B tends to diminish the competitive advantage of the viral RNA (18). (According to the Basel group there is a requirement for eIF-4B for initiation on EMC RNA (17) Ihe conclusion which is the most consistent with all these observations is therefore that the optimiam ratio of eIF-4B to other factors may be lower for EMC ENA than for cellular mRNAs), The shut-off of host cell protein synthesis in this view is due to the competition by the viral RNA at the initiation step. The competitive effect might be enhanced if viral infection resulted in some inactivation or modification of eIP-4B, and some compelling evidence for this has recently been found (19)> althou the nature of the change in eIP-4B remains unknown. 

Both of these ideas have their attractions, but they seem unable to account for all virus-host systems, particularly those in which the reduction in host cell protein synthesis occurs soon after infection. The increased permeability of the membrane to monovalent cations is generally thought to occur too late in the infection, and the out-competition by viral mRNA can only cause a significant reduction in host cell protein synthesis when viral mRNA has been produced in sufficient amounts to suppress initiation on host mRNA. As a total explanation, the competition hypothesis seems limited to those systems where the overall rate of protein synthesis remains relatively constant after infection, the rate of host protein synthesis declining progressively as the rate of viral protein synthesis increases. It cannot, for instance, explain the rapid reduction in host protein synthesis which occurs shortly after... 

HELENTJARIS, T. and EHRENFELI), E. Inhibition of host cell protein synthesis by UV-inactivated poliovirus. J. Virol. (I977)f... 

CELMA, M.L. and EHEIENEELD, E. Effect of poliovirus double-stranded RNA on viral and host cell protein synthesis. Proc. Nat. Acad. Sci. U.S.A. (1974), H, 2440-2444. 

Schnitzlein, W. M., O Banion, M. K., Poirot, M. K., and Reichmann, M. E., 1983, Effect of intracellular vesicular stomatitis virus mRNA concentration on the inhibition of host cell protein synthesis, J. Virol. 45 206. 

Baglioni, C., Simili, M., and Shafritz, D. A., 1978, Initiation activity of EMC virus RNA, binding to eIF-4B and shut-off of host cell protein synthesis. Nature 275 240. 

Svitkin, Y. V., Ginevskaya, V. A., Ugarova, T. Y., and Agol, V. I., 1978, A cell-free model of the encephalomyocarditis virus-induced inhibition of host cell protein synthesis. Virology 87 199. 

Picornavirus Inhibition of Host Cell Protein Synthesis... 

Infection of cultured cells with many lytic viruses results in a marked decrease in the rate of cellular protein synthesis. Usually, this decrease is accompanied by increasing rates of viral protein synthesis, marked cytopathic effects, and ultimately cell death. In most cases, it is not known whether the shut-off of host cell protein synthesis results from an active process induced by the virus evolved for that (or some other) purpose, or whether it is merely a passive result of another viral function, such as production of large quantities of viral mRNA which compete effectively with their cellular counterparts. In the case of poliovirus, however, three types of studies suggested that the former, active type of mechanism was at work. Kinetic analysis of the rate of protein synthesis in cells synchronously infected with high multiplicities of virus showed that cellular protein synthesis could be virtually completely inhibited prior to the synthesis of significant quantities of viral RNA and protein (Summers et ai, 1965). In addition, infection in the presence of 1-3 mM guanidine, which prevents detectable replication of viral RNA, nevertheless results in viral inhibition of host cell protein synthesis (Holland, 1%4 ... 

PROPERTIES OF INHIBITION OF HOST CELL PROTEIN SYNTHESIS BY POLIOVIRUS... 

One approach was to prepare extracts of poliovirus-infected HeLa cells and to assay for an inhibitor of initiation by addition to a reticulocyte lysate translation reaction. The reticulocyte lysate had been well-characterized as a system capable of efficient initiation in vitro. A potent inhibitor of initiation was indeed found in the cytoplasm of poliovirus-infected cells, which was not present in similar preparations from uninfected HeLa cells (Hunt and Ehrenfeld, 1971), and the inhibitor was subsequently identified as double-stranded poliovirus RNA (Ehrenfeld and Hunt, 1971). In order to qualify as a specific inhibitor of host cell protein synthesis, however, it was necessary to demonstrate that viral protein synthesis was immune to the inhibitory effects of the putative shut-off factor. When double-stranded RNA was put to this test, no specificity between cellular and viral protein synthesis could be demonstrated (Celma and Ehrenfeld, 1974) rather, all protein synthesis was inhibited equally. In addition, concentrations of viral double-stranded RNA required to... 

An alternative type of explanation for the specific discrimination against host cell protein synthesis in poliovirus-infected cells stemmed from the observation that initiation of protein synthesis could be selectively inhibited in HeLa cells and in poliovirus-infected HeLa cells by increasing the osmolarity of the growth medium (Sa-borio et al., 1974). The inhibition was independent of the solute used to increase the osmolarity. However, virus-directed protein synthesis was observed to be relatively more resistant to inhibition by hypertonic medium than was cellular protein synthesis, a fact which was interpreted as indicating that initiation of viral RNA translation was intrinsically more efficient than that of cellular mRNA (Nuss et al., 1975). These workers, therefore, proposed that the virus-specific or virus-induced factor involved in suppression of host protein synthesis could function by indiscriminantly lowering the rate of peptide chain initiation. Under such conditions, translation of viral mRNA, when it was synthesized, could occur due to its inherently strong affinity... 

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